The valve-regulated lead-acid battery has been widely used in power supply systems of industries like electricity, communications, transportation, finance and army, as back up in substation, machine room, mobile base station, UPS and start up for generators. It is used to provide continuous electricity supply to the electrical load when the commercial power has power supply failure or the commutator breaks down. Therefore the battery plays a very important role in the power supply system.
VRLA is a complicated electrochemical system whose service life and performance are related to many factors including the material of electrodes, techniques, the change of active material and operating situation, etc, and it is also one of the parts of the equipment which is easy to break down. In general, the cause of the performance decrease of VRLA can be briefly classified as follows: dehydration, sulfation, corrosion and deformation of grid, softening of active material and short circuit of dendrite, etc.
The current exiting methods of measuring and evaluating the battery performance:
There are several methods of battery testing, currently used as follows: checking discharge testing, in-process voltage checking, battery internal resistance (conductance) testing and discharge capacity estimating.
1. Checking Discharge Testing
As the standard of battery capacity, checking discharge testing is the most standard and effective performance testing method for the battery, which can completely reflect the battery capacity. It can be used online or offline to measure.
Unfortunately, checking discharge testing has great workload, inconvenient operation and potential danger to the system when doing online testing. This method is not suitable to be used frequently due to the limited battery cycle life. It also does not have the ability of real-time monitoring for the battery working environment.
2. In-Process Voltage Checking
Among the current existing battery testing equipments, the common online monitoring device is in-process checking instrument for a battery set, generally used for online monitoring the voltage, current and temperature of a single battery to judge whether the accumulator is good or bad according to the voltage.
Lots of practical experience shows that, the battery voltage (float charge or equalized charge) during operation almost has no relevance to the battery capacity. In actual practice, we generally use it as the observation and record of battery charge curve or battery float charge data.
3. IR (Conductance) Testing of the Battery
In the 1992 INTELEC Dr. David Feder delivered a thesis on the relevance between conductance and battery capacity which brought up the importance of the conductance measurement to the battery testing. Because of its convenience, its function of finding failure battery and its sensitivity of IR increase due to battery dehydration, the IR or conductance testing method has important reference value of finding out failure battery in time and grasping the change of the battery performance.
However, it is a great pity that the battery capacity doesn't have good relevance to the battery conductance. When the battery capacity is above 80%, the battery capacity almost has no relevance to the battery conductance. Most of battery having linear relation with capacity are failure ones whose capacity is below 80%. Therefore, the value of actual measurement is lost.
4. Discharge Capacity Estimating
It estimates the residual capacity of the battery through 5-15 minutes' load discharge. The theoretical basis of this testing method is comparing to the experiential discharge curves. However, when the battery performance degrades seriously, the discharge curve will change a lot which will lead to the serious deviation of the predict value. Meanwhile, because of the different battery manufacturers, and the batteries themselves are not the same, which will also lead to the deviation of the estimating. Therefore this method is seldom used now.
To sum up, the current battery performance evaluations apply a single method to predict the battery performance at a single time and from a single perspective. Yet it is a very complex process for a battery to change from good to bad and the evaluation of a single model is insufficient. This is the reason why there is no convenient, valid and accurate method to measure the capacity and performance of the battery till now.